A known nonpeptide .delta. opioid antagonist is naltrindole, which is described by P. S Portoghese, et al J. Med. Chem. 31, 281-282 (1988). Naltrindole has similar .delta.-antagonist potency as the compounds according to this invention but is much less .delta. selective. Furthermore, naltrindole has also quite high .mu. opioid receptor affinity (K.sub.i.sup..mu. =12 nM) in the receptor binding assay and potent antagonist properties (K.sub.e =29 nM) in the guinea pig ileum (GPI) assay, cf P. S. Portoghese, J. Med. Chem. 34, 1757-1762 (1991).
Another known .delta.-antagonist is the enkephalin analog N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174864) (SEQ ID NO: 1 in the Sequence Listing) described by R. Cotton, et al. in Eur. J. Pharmacol. 97, 331-332 (1984). In comparison with the .delta. antagonists described in this patent application, ICI 174864 is much less .delta.-selective (10-300 times less) and has much lower antagonist potency in the MVD assay (40-1000 times less potent).